This invention relates to an electrical bushing and more particularly to improvements in tip construction for an electrical bushing that is used at the lead portion of an electrical device.
FIG. 1 shows a sectional view of a tip portion of a conventional bushing for high voltage use and used as the lead portion of an electrical device. This is a so-called center clamping type oil-filled bushing that generally utilizes the central conductor as a clamping member. In the drawing, this conventional bushing comprises a hollow insulating tube 1, a central conductor 2 extending through the tube and a resilient member 3 supplying a compressive force to the tube 1 and which in this figure is a spring concentrically disposed around and insulated from the conductor 2. The load from the resilient member is received by a step 4. Further, the bushing comprises a terminal fitting 5 fixing the step 4 to the central conductor 2, insulating oil 6 sealed within the bushing, an expansion chamber housing 7 which absorbs expansion and contraction of the insulating oil and alleviates changes in the inner pressure, a terminal cap 8 fixed to the expansion chamber housing 7, flexible conductor 9 one end of which is connected to the terminal fitting 5 and the other end thereof being connected to the terminal cap 8 in order to conduct current of the central conductor outside of the bushing. Furthermore, the bushing comprises a bottom plate 10 which is disposed between the resilient member 3 and the hollow insulating tube 1 for fixedly holding the electrically insulating expansion chamber housing 7 which is bolted thereto, an inert sealing gas filled within the expansion chamber housing 7 and a gasket 12 for maintaining a seal between the expansion chamber housing 7 and the terminal cap 8.
The operation of such conventional apparatus will hereinafter be explained. Due to changes in external temperatures and in the current flowing in the central conductor 2, temperature changes occur in the central conductor 2 and the hollow insulating tube 1 disposed surrounding the conductor 2, resulting in the expansion and contraction thereof. However, since the central conductor 2 has a coefficient of thermal expansion different from the hollow insulating tube, it contracts and expands in relation to the hollow insulating tube 2. Therefore, the flexible cnductor 9 is used to absorb the displacement between the expansion chamber housing 7 which is fixedly held by the bottom plate 10 press fit against the hollow insulating tube 1 by the resilient member 3 and also to conduct current outside of the housing 7. This flexible conductor 9 has one end connected to the terminal fitting 5 mounted to the central conductor 2 and the other end attached to the terminal cap 8. This is done by pulling the flexible conductor 9 out through an opening in the expansion chamber housing 7 used for mounting cap 8, securing the one end to cap 8, and then mounting the terminal cap 8 to the housing 7 using a gasket 12.
FIG. 2 is a cross sectional view of another example of a bushing. In the drawing, the same numerals are used to represent the same or corresponding portions as in FIG. 1. The bushing comprises a terminal spindle 13 screwed into the central conductor 2 and a stopper 14 for transmitting the load of the resilient member 3 through the step 4 to the terminal spindle 13. A terminal cap 8 is fixed such as by welding to the expansion chamber housing 7 and a movable terminal 15 is fixed to the terminal spindle 13 and presses against the side of the terminal cap 8 to connect therewith. In conventional bushings of this type, the above elements can be assembled before the expansion chamber housing 7 is mounted; and when the housing 7 is finally fixed, the terminal cap 8 and the movable terminal 15 are electrically connected to each other, which is an extremely good feature in terms of assembly. However, both of these examples have respective inherent problems.
In the former example shown in FIG. 1, the weight of the flexible conductor 9 becomes heavier for large currents, thus deteriorating performance. Further, since a good seal is required from the gasket 12 between the terminal cap 8 and the housing 7 and since analysis of the inner insulating oil and measurement of the inner pressure are generally not carried out during periodic maintenance checks while the bushing is in use, a bad seal resulting from degeneration of the gasket 12 is generally not discovered until damage has taken place. In the worst case, the penetration of rain water and the resulting deterioration of the insulating oil 6 can lead to a serious accident such as dielectric breakdown.
The problem with the latter example shown in FIG. 2 is that due to the expansion/contraction of the central conductor 2, there is a fear of metallic powder being generated from the resulting rubbing between the movable terminals 15 and the terminal cap 8, such powder mixing into the insulating oil possibly leading to a significant dielectric breakdown accident.